DIGESTIVE SYSTEM : ORAL CAVITY-ENGLISH
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DIGESTIVE SYSTEM : ORAL CAVITY
• The
digestive system is also known as the gastrointestinal system or the alimentary
canal or GIT. This vast system is
approximately 10 m long. It travels the length of the body from the mouth
through the thoracic, abdominal and pelvic cavities, where it ends at the anus.
We will cover it in multiple lectures. Today we are going to discuss the
anatomy and physiology of the oral cavity.
ORAL CAVITY
• The
mouth or oral cavity is first organ of GIT . It is surrounded-
• by
the lips-Anteriorly
• by
oropharynx - Posteriorly
• by
the muscles of the cheeks-Laterally
• by
the bony hard palate and muscular soft palate-Superiorly
• by
the muscular tongue and the soft tissues of the floor of the mouth-Inferiorly.
• The
oral cavity is lined throughout with mucous membrane, consisting of
stratified squamous epithelium containing small mucus-secreting glands. The lips
and cheeks are formed of muscle and connective tissue. This allows
the lips and cheeks to move food mixed with saliva around the mouth and begin mechanical
digestion. The teeth contribute to mechanical digestion by grinding and
tearing food. This process of chewing and mixing food with saliva is called mastication
• The roof of the mouth is formed by palate which
is divided into the hard palate and
the soft palate. The hard palate is formed by the maxilla and the
palatine bones. The soft palate is muscular, curves downwards from the
posterior end of the hard palate . The uvula is a curved fold of muscle
covered with mucous membrane, hanging down from the middle of the free border
of the soft palate. Posterior end of oral cavity contains palatine tonsils
TONGUE
The tongue is a large, voluntary muscular structure that
occupies much of the oral cavity. It is attached posteriorly to the hyoid bone
and inferiorly by the frenulum. The superior surface of the tongue is
covered in stratified squamous epithelium for protection against wear and tear.
This surface also contains many little projections called papillae or taste
buds. The taste buds contain sensory
receptors (specialised nerve endings) for the sense of taste .
There are four types
of papillae on the toungue- vallate papilla, follate papilla, filliform
papilla and fungiform papilla. Apart
from the sense of taste, other functions of the tongue include swallowing
(deglutition), holding and moving food
around the oral cavity and speech.
TEETH-
The teeth are the hardest substances in the human body.
Besides being essential for chewing, the teeth play an important role in
speech. The teeth are embedded in the alveoli or sockets of the alveolar ridges
of the mandible and the maxilla. Babies are born with two sets, or dentitions,
the temporary or deciduous teeth and the permanent teeth .
At birth the teeth of both dentitions are present, in immature form, in the
mandible and maxilla.
• Temporary
teeth are also known as deciduous teeth or milk teeth. Temporary teeth begin to
appear at about 6 months old. There are 20 temporary teeth, and these are
replaced by permanent teeth from about the age of 6 years .There are 32 permanent teeth. 16 are located
in the maxilla arch (upper) and 16 are located in the mandible
TYPES OF TEETH
There are 4 types of teeth depending upon shape and
function-
Incisors – The four front teeth in both the
upper and lower jaws are called incisors. Their primary function is to
cut food. The two incisors on either side of the midline are known as
central incisors. The two adjacent teeth to the central incisors are
known as the lateral incisors. Incisors have a single root and a sharp
incisal edge.
Canines – There are four canines in the oral
cavity. Two in the maxillary arch and two in the mandibular area. They
are behind and adjacent to the lateral incisors. Their main function is
to tear food. They have a single, pointed cusp and a single root.
They have the longest root of any tooth.
Premolars – These teeth are located behind and
adjacent to the canines and are designed to crush food. There are eight
premolars in the oral cavity. There are two in each quadrant of the
mouth. These teeth can have 3-4 cusps. The maxillary first premolar
has two roots, and the remaining premolars have a single root. There are
no premolars in the primary dentition
Molars – The most posterior teeth in the
mouth are the molars. They have broader and flatter surfaces with 4-5
cusps. They are designed to grind food. Mandibular molars typically
have 2 roots. Maxillary molars have 3 roots. There are 12 molars in
the permanent dentition with three in each quadrant of the mouth. The primary dentition only contains eight
molars.
STRUCTURE OF THE TEETH-
• Although
the shapes of the different teeth vary, the structure is the same and consists
of:
• the
crown – the part that protrudes from the gum
• the
root – the part embedded in the bone
• the
neck – the slightly narrowed region where the crown merges with the root.
In the centre of the tooth is the pulp cavity containing
blood vessels, lymph vessels and nerves, and surrounding this is a hard
ivory-like substance called dentine. Outside the dentine of the crown is
a thin layer of very hard substance, the enamel. The root of the tooth,
on the other hand, is covered with a substance resembling bone, called cement,
which secures the tooth in its socket. Blood vessels and nerves pass to the
tooth through a small foramen (hole) at the apex of each root
SALIVERY GLAND-
Salivary glands release their secretions into ducts that
lead to the mouth. There are three main pairs:
• The
parotid glands,
• The
submandibular glands and
• The
sublingual glands.
• Parotid
glands- These are situated one on each side of the face just below the
external acoustic meatus. Each gland has a parotid duct opening into the
mouth at the level of the second upper molar tooth.
• Submandibular
glands-These lie one on each side of the face under the angle of the jaw.
The two submandibular ducts open on the floor of the mouth, one on each
side of the frenulum of the tongue.
• Sublingual
glands- These glands lie under the mucous membrane of the floor of the
mouth in front
• Saliva
is the combined secretions from the salivary glands and the small
mucus-secreting glands of the oral mucosa. About 1.5 litres of saliva is
produced daily and it consists of:
• -Water -Mineral
salts
• -Mucus -Lysozyme
• -Immunoglobulins -Blood-clotting
factors.
• -A
digestive enzyme: salivary amylase
FUNCTIONS OF SALIVA-
Chemical digestion of polysaccharides- Saliva
contains the enzyme amylase that begins the breakdown of complex sugars,
including starches, reducing them to the disaccharide maltose. Enzyme action continues during swallowing
until terminated by the strongly acidic pH (1.5 to 1.8) of the gastric juices,
which degrades the amylase.
Lubrication of food- The high water content means
that dry food entering the mouth is moistened and lubricated by saliva before
it can be made into a bolus ready for swallowing.
Cleaning and lubricating the mouth- An adequate
flow of saliva is necessary to clean the mouth, and to keep it soft, moist and
pliable. It helps to prevent damage to the mucous membrane by rough or abrasive
food.
Non-specific defence- Lysozyme, immunoglobulins
and clotting factors present in saliva combat invading microbes.
Taste- The taste buds are stimulated only by
chemical substances in solution and therefore dry foods stimulate the sense of
taste after thorough mixing with saliva.
FUNCTIONS OF THE MOUTH-
Formation of a bolus- When food is taken into the
mouth it is chewed (masticated) by the teeth and moved round the mouth by the
tongue and muscles of the cheeks. It is mixed with saliva and formed into a
soft mass or bolus ready for swallowing. The length of time that food remains
in the mouth depends on the consistency of the food. During this process
digestion by salivary amylase continues. Some foods need to be chewed longer
than others before the individual feels that the bolus is ready for swallowing.
Swallowing (deglutition) - This
occurs in three stages after chewing is complete and the bolus has been formed.
It is initiated voluntarily but completed by a reflex (involuntary) action. The
mouth is closed and the voluntary muscles of the tongue and cheeks push the
bolus backwards into the pharynx. The muscles of the pharynx are stimulated by
a reflex action initiated in the walls of the oropharynx and coordinated in the
medulla and lower pons in the brain stem. Involuntary contraction of these
muscles propels the bolus down into the oesophagus
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